The present invention relates to regioselective synthesis of acyclonucleosides and their derivatives.
Guanine-related acyclonucleosides with the purine substitution at the 9-position have been recognized as potentially important antiviral agents. Unfortunately, it was difficult to prepare such compounds without alkylation (glycosylation) occurring at the 7-position as well; for example, see Ogilivie et al., Can. J. Chem., 60, 3005-3010 (1982) and Martin et al., J. Med. Chem., 26, 759-61 (1983) for 7/9 mixtures which are generally difficult to separate.
We have discovered a novel process for preparing 9-substituted guanine-related acyclonucleosides which prevents or minimizes alkylation at the 7-position and which furnishes 7- and 9- isomers which are much more easily separated. This method is also generally applicable to other purines. The word "alkylation" is used here to refer to attachment of an alkyl or cycloalkyl substituent containing protected or unprotected hydrophilic groups, where the carbon at the attachment site can be an alkyl carbon or else a carbon at the aldehyde oxidation state, such as an acetal or aminal carbon. The words "guanine-related" shall be understood to mean 2,6-disubstituted purines where the 2- and 6-subtitutuents are amino or protected amino and hydroxy or protected hydroxy, respectively, and groups which can be converted to the aforementioned groups by nucleophilic substitution, for example, displacement of halide ion from 2-amino-6-choropurine. Thus, "guanine-related" includes compounds such as 2-amino-6-benzyloxypurine; 2,6-dichloropurine; 2-bromohypoxanthine; 2,6 diazidopurine;-2,6-dichloropurine; 2-acetamido-6-chloropurine; 2-acetamidohypoxanthine; and 2-isobutyramidohypoxanthine.
The process of the present invention comprises alkylation of pro-guanine derivatives having a bulky and hydrophobic blocking group at the 6-position of the purine and optionally at the 2-position of the purine. When appropriate in the synthesis, the blocking group (or groups) may be removed by standard methods (for example, by hydrogenolysis or treatment with acid, e.g. dilute hydrochloric acid, dilute sulfuric acid or trifluoroacetic acid, or by nucleophilic placement, followed by hydrolysis or .beta.-elimination). The words "pro-guanine derivative" shall be understood to mean a purine derivative which can be converted by deprotection or nucleophilic substitution to guanine or a 9-substituted guanine derivative such as 2-amino- 6- benxyloxypurine or 2-chloro-6-benzyloxypurine or 2-isobutylamido-6-(p-nitrophenylethoxy)purine.
Any blocking group that will effectively prevent or minimize alkylation at the 7-position may be used in the 6-position of the pro-guanine derivative. Examples of suitable blocking groups are 6-benzyloxy, substituted 6-benzyloxy, 6-phenoxy, substituted 6-phenoxy, 6-(2-phenylethoxy), substituted 6-(2-phenylethoxy), especially 6-[2-(4-nitrophenyl)ethoxy]; aryl- and substituted arylsulfonyloxy, and 6-(aryl-and/or alkyl-siloxy); and 6-(.beta.-cyanoethoxy). As used herein aryl means phenyl, naphthyl, substituted phenyl or substituted naphthyl. The aforementioned benzyloxy, phenoxy, phenylethoxy, aryl and phenyl groups may be substituted on the phenyl or naphthyl moieties with substituents selected from C.sub.1 to C.sub.6 alkyl, halo (i.e., fluoro, chloro, bromo and iodo), nitro, phenyl, and trifluoromethyl.
The pro-guanine derivative is alkylated with a suitable alkylating agent in an inert solvent at a temperature of -80.degree. to 150.degree. depending on the reactivity of the alkylating agent and the nature of the solvent. Atmospheric pressure is preferred but higher or lower pressures may be used, if desired. Alkylation may also be accomplished in the presence of a strong base such as sodium hydride, potassium carbonate, triethylamine, or lithium diisopropylethylamide.
Optionally the pro-guanine heterocyclic derivative may be trimethylsilylated to facilitate solution and increase yields.
The following reaction schemes illustrate the process of the present invention: ##STR1## X=halogen, tosylate, acetate, or other appropriate leaving group R.sup.1 =blocking group
R.sup.2 =NH.sub.2, Cl, Br PA1 R.sup.3 =protected, side-chain derivative ##STR2## PA1 (c) 20% Pd (OH).sub.2 on carbon, H.sub.2 (50 psi), EtOH PA1 (d) 20% Pd (OH).sub.2 on carbon, H.sub.2 (50 psi), TsOH, EtOH/H.sub.2 O PA1 (e) NaIO.sub.4 in H.sub.2 O PA1 (f) NaBH.sub.4 PA1 (g) HAc-HCl (20:3 v/v), 55.degree.-60.degree., 11/2 hours, or CF.sub.3 CO.sub.2 H: H.sub.2 O 1:9, room temperature, overnight. PA1 X is a suitable leaving group. PA1 R.sup.3 is amino;
As showm in Scheme II, the process of the present invention may be used to prepare (S)-9-(2,3-dihydroxy-1propoxymethyl)guanine: ##STR3## (a) CH.sub.2 O, HCl (g), CH.sub.2 Cl.sub.2, O.degree. (b) 2-amino-6-benzyloxypurine, NaH, DMF, room temperature
Methyl 2,3,4-tri-O-benzyl-.alpha.-D-glucopyranoside is prepared in two steps from the commercially available methyl .alpha.-D-glucopyranoside; see R. Eby and C. Schuerch, Carbohydrate Res. 34, 79 (1974). The compound was chloromethylated at the 6-position using paraformaldehyde and HCl gas in CH.sub.2 Cl.sub.2 as solvent. (Caution: Bis-chloromethyl ether, a potent carcinogen, is presumably formed as a by-product in this reaction and the procedure should be carried out in a well ventilated hood). The product, methyl 2,3,4-tri-O-benzyl-6-O-chloromethyl-.alpha.-D-glucopyranoside, is used to alkylate 2-amino-6-benzyloxypurine. The product, methyl 2,3,4-tri-O-benzyl-6-O-(2-amino-6-benzyloxypurin-9-ylmethyl)-.alpha.-D-glu copyranoside, is obtained after a silica gel column separation. Debenzylation is carried out by hydrogenation over 20% Pd(OH).sub.2 on carbon. In this deblocking step, the presence or absence of p-toluenesulfonic acid determines the nature of the product formed. Thus, when the acid is omitted, debenzylation of the heterocycle occurs leaving the blocking groups on the sugar moiety intact. In this way, the intermediate may be readily isolated. If 3 molar equivalents of p-toluenesulfonic acid are added to the hydrogenation, complete deblocking occurs to give methyl 6-O-(guanin-9-ylmethyl)-.alpha.-D-glucopyranoside.
The methyl 6-O-(guanin-9-ylmethyl)-.alpha.-D-glycopyranoside, is dissolved in water and treated with sodium periodate (3 molar equivalents). After removal of excess periodate by precipitation with strontium chloride, the intermediate dialdehyde 8 is not isolated but rs reduced immediately with sodium borohydride to give the presumed (2S,l'S)-2-O(2'-hydroxy-1'-methoxyethyl)-1-O-(guanin-9-ylmethyl) glycerol 9. Acidic hydrolysis of 9 with HAc-HCl or with aqueous CF.sub.3 COOH gives the required 10.
Other compounds that may be prepared by the process of the present invention are cyclic phosphates of 2,6-substituted purines.
Acyclonucleoside cyclic phosphates are disclosed in European patent application No. 82401571.3, publication No. 0 074 306, U.S. Ser. No. 538,019, filed Sept. 30, 1983 and U.S. Ser. No. 616,910, filed June 6, 1984. These compounds were prepared by direct phosphorylation of an acyclonucleoside which contained two hydroxyl groups. Yields were poor because in solvent systems where the acyclonucleosides had measurable solubility, phosphorylation of the second hydroxyl was able to compete successfully with the cyclization reaction of the first phosohorylation. Using the process of the present invention, a phosphorylated side chain is separately synthesized and this side chain is used to alkylate a suitably substituted purine or pyrimidine.
Thus, in one of its embodiments the present invention relates to a process for preparing cyclic phosphates of 2,6-substituted purines comprising alkylating a purine having a blocking group at the 6-position with a compound of the formula: ##STR4## wherein R.sup.1 is alkyl of 1 to 18 carbons, haloalkyl of 1 to 18 carbon atoms, benzyl, substituted benzyl, phenyl or substituted phenyl, wherein halo means fluorine, chlorine, bromine or iodine and the substituents on the phenyl group or the phenyl moiety of the benzyl group are selected from alkyl, nitro and halogen; and
Preferably X is halide (i.e., fluorine, chlorine, bromine or iodine) or tosyloxy;
More preferably X is chloride.
Preferably, R.sup.1 is o-chlorophenyl.
Preferred pro-guanine derivatives are 2-amino-6-benzyloxypurine, 2-chloro-6-benzyloxypurine, 2- amino-6-o-nitrophenoxypurine, 2-amino-6-[2-(4-nitrophenyl)ethoxy]purine, 2-amino-6-.beta.-cyanoethoxypurine, 2-amino-6-chloropurine, and 2,6-dichloropurine. Most preferably the pro-guanine derivative is 2-amino-6-benzyloxypurine.
A preferred embodiment of the present invention relates to a process for preparing a compound of the formula: ##STR5## wherein R.sup.2 is benzyloxy; and
comprising alkylating a compound of the formula: ##STR6## wherein R.sup.2 and R.sup.3 are as defined above with a compound of the formula: ##STR7## wherein R.sup.1 is as defined above and X is a suitable leaving group. Preferably X is halide or tosyloxy. More preferably, X is chloride.
The following reaction scheme illustrates this process. ##STR8##
2,5-Methylene-D-mannitol (11) is converted by a multi-step procedure to the Bis-protected-1,3,2-dioxaphosphorinan-5-yloxy P-oxide derivative (12) which is converted by formaldehyde and hydrogen chloride to the reactive chloromethyl ether derivative 13. Alkylation by 13 of the pro-guanine derivative, 2-amino-6-benzyloxy-purine, either as the silyl derivative in an inert solvent such as acetonitrile, tetrahydrofuran, benzene or toluene or directly in the presence of a strong base such as sodium hydride in dimethylformamide, gives predominantly the 9-substituted product 14 which can be deprotected at the 6-position by hydrogenolysis and/or aqueous acid treatment.